Screens in accordance with above are used for protection of plants outdoors and inside greenhouses, for adjustment of the in- and outflow of radiant energy, so that the plants are protected against too strong sunlight and against too high or too low temperatures. These screens extend vertically and/or horizontally over very large areas, for instance set-ups with up to 30 000 m.sup.2 occur. Such a screen set-up protects large values, since plants may represent a value of about 100 US dollar per square meter. Electrical motors are used to maneuver the screens between active and inactive positions. Such a greenhouse screen is described in EP 0109951 and comprises narrow strips of plastic foil, which strips are held together by textile threads and may consist of a polyester foil being metallized on one of its sides.
Fires can start in screen systems e.g. because of defects in electrical systems, by repairings such as welding, or when a vehicle is driven inside the greenhouse under a screen system. It has been shown that these fires can spread rapidly over large areas via the screens. Such fires may injure people working in the greenhouse and damage the greenhouse in itself. Besides, the vegetation which the screen is meant to protect, is destroyed.
It has also proven hard to modify known plant protection screens, so that they become flameretardant with maintained protection against UV-radiation, during a life expectancy of at least four to five years, without at the same time being very bulky in a drawn aside position.
Flameretardant textiles are used in certain environments where one wants to limit the risk of a small local seat of fire being spread over a larger area.
Flameretardant yarn textiles are complicated to produce, because of the long way of manufacturing. Such known textiles are not heat; reflecting, transmit light to a low extent, are dirt accumulating and water absorbing. Moreover, they do just partly reflect solar radiation. The production cost is high and the width limited. These flameretardant textiles are not usable in connection with greenhouses.
Lately, the technical evolution in the area of plastic film has reached so far, that the flameretardancy is satisfied, but the big problem remains, to overcome and slow down the ageing phenomenon, so the user can be guaranteed a definite life of the plastic film. The difficulties have been, that the addition of halogen, in order to improve the flameretardancy, has taken place at the expense of the UV-stability, which was reduced to such an extent that the material was falling to pieces, either already by production or shortly after being exposed to UVrays.
AU-A-79349/91 describes a flameretardant laminate to be used as insulation layer and diffusion barrier within the building sector. This laminate can at its most be used as roller blind, but it does not meet the demands for a greenhouse screen, since the laminate is waterproof, which means that large pools of water may be formed on the horizontal parts of the greenhouse screen, for which water load the screen system is not dimensioned. Condensation water can be gathered at the underside of the laminate and drip down onto the plants, which then get damaged. Nor does the laminate possess the necessary characteristics, i.e. the quality to be able to be draped, which is required for being able to displace the curtain sideways and park it in the form of a narrow package, which steals a minimum of incoming light.
Polymers with a chemical structure which reduces the combustibility have so far been expensive to produce. For plant protecting screens, there are also demands on providing products having different degrees of light permeability, which put higher demands on the UV-protection.
The demand of certain characteristics, such as the ability of being draped, sideways displaced and easy mounted, in combination with demands on low weight and small volume have also resulted in, that the thickness of the materials being part of the curtain, which to a large extent consists of strips, must be very thin. This demand is very hard to combine with, partly the desired long-time UV-stabilization and the demand on being difficult to ignite, UV stabilization becomes easier to fulfill the thicker the plastic film of the laminate is (the filter effect). In the demand on being difficult to ignite, the amount of material influences the self-extinguishing ability. A thin material contains a small amount of fire protection means.